math/vector: constexpr doc cleanup.
math/vector: constexpr doc cleanup.

Everything that can be constexpr in C++11 is now constexpr.

The docs now reflect this.

After some research, I've decided that the 'inline' keyword is redundant
when 'constexpr' is present, and removed cases of it here.

cbiffle/etl,cbiffle/etl,cbiffle/etl